<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        *{
            margin: 0px;
            padding: 0px;
        }
        html,body{
            height: 100%;
        }
        .floatL{
            float: left;
            width: 200px;
            height: 100%;
            background-color: chartreuse;
        }
        .overF{
            overflow: hidden;
            width: auto;
            background-color: coral;
            height: 100%;
        }
        .active {
            width: 150px;
        }
        .position {
            position: absolute;
            left: 150px;
            top: 50px;
        }
    </style>
</head>
<body>
    <div class="floatL">
        <ul class="active active1">
            <li class="ul_li">
                一级导航1
                <ul class="active active2 position position1">
                    <li>二级导航a1</li>
                    <li>二级导航a2</li>
                    <li>二级导航a3</li>
                </ul>
            </li>
            <li class="ul_li">
                一级导航2
                <ul class="active active2 position position2">
                    <li>二级导航b1</li>
                    <li>二级导航b2</li>
                    <li>二级导航b3</li>
                </ul>
            </li>
            <li class="ul_li">一级导航3</li>
            <li class="ul_li">一级导航4</li>
            <li class="ul_li">一级导航5</li>
        </ul>
    </div>
    <div class="overF">

    </div>
</body>
</html>